A Gilbert Cell type variable gain amplifier (hereinafter referred to simply as a “differential amplifier”) may be used as a circuit for driving an optical modulator in an optical transceiver or the like. The differential amplifier has a variable gain which can be varied in accordance with a gain control signal given to bases of four transistors provided in an upper stage (see, for example, Japanese Unexamined Patent Publication No. 2011-205470). Proper adjustment of the gain control signal with respect to a fluctuation factor such as temperature, for example, allows the optical modulator driven by an output signal of the differential amplifier to stably generate an optical output signal having a constant amplitude. However, when the gain is changed, an average potential (center potential) of the output signal undesirably moves to cause malfunction of the amplifier in the next stage or distortion of the output signal. Further, in a case in which a current source of the differential amplifier is set to provide a large source current enough to suppress distortion with respect to a maximum input amplitude, a smaller input amplitude decreases power efficiency of the differential amplifier.